Cathode ray tubes are widely used in display apparatus, such as television receivers, for displaying an image. In order to generate the image, a potential difference must be developed between the cathode and grid electrodes of the CRT to stimulate the emission of an electron beam from the cathode. When the potential difference is varied as a function of a video signal, the intensity of the electron beam varies accordingly and an image representative of the video signal is developed on the CRT screen.
In color television receivers, two types of driver circuits are generally used to develop the cathode-grid potential difference, both described in Basic Television and Video Systems by B. Grob, 5th edition, McGraw-Hill, 1984, p. 117-118 and 177. These two types of circuits use the R, G and B (red, green and blue) signal components of the incoming television signal, and the Y signal component (luminance signal) in different ways. In the first type of circuit, control signals representative of R-Y, G-Y and B-Y color difference signals are applied to three separate CRT control grids and a control signal representative of the -Y signal is applied to the three CRT cathodes. The CRT acts as a mixer to provide potential differences between respective cathode and grid electrode pairs, the potential differences varying as a function of the R,G and B video signals respectively. This driving arrangement has the disadvantage of establishing a grid electrode with a high negative potential, which too frequently results in an arcing problem within the CRT envelope.
In the second type of driving circuit a matrix circuit is used to remove the Y component from the color difference signals. Control signals representative of the resulting R, G and B video signals are applied to the respective CRT cathodes, while the grid of each electrode pair is grounded to produce the required potential difference. This single-ended type of driving circuit has the inherent disadvantage of requiring high voltage control signals to drive the cathode.
It is well known to those skilled in the art that the selection of amplifier circuit components normally requires a trade-off between voltage capacity and frequency response, the tradeoff being necessitated by inherent limitations in the amplifying devices. Particularly when it is desired to use semiconductor amplifiers in high frequency applications, such as video amplifiers, the tradeoff often makes it difficult to select circuit components which are capable of developing the required drive voltages at video frequencies.
The frequency response problem inherent in using semiconductor amplifiers in high voltage CRT cathode driver circuits has been recognized, as shown in U.S. Pat. No. 4,233,624 to Hinn et al. The Hinn patent addresses the problem through the use of a filter circuit to detect high frequency video signal components, compensating and combining circuits to remove the high frequency video components from the cathode driver circuits, and amplifier circuits to apply those components to the grid. Hinn's CRT driver circuit, however, requires a high voltage, high frequency amplifier in the grid driver circuit and is thus subject to the disadvantages attending such an arrangement. Further, Hinn's circuit requires complex filtering and combining circuitry.